f2fs: avoid build warn of fall_through

f2fs: avoid build warn of fall_through

After merging the f2fs tree, today's linux-next build
 (x86_64_allmodconfig) produced this warning:

 In file included from fs/f2fs/dir.c:11:
 fs/f2fs/f2fs.h: In function '__mark_inode_dirty_flag':
 fs/f2fs/f2fs.h:2388:6: warning: this statement may fall through [-Wimplicit-fallthrough=]
    if (set)
       ^
 fs/f2fs/f2fs.h:2390:2: note: here
   case FI_DATA_EXIST:
   ^~~~

 Exposed by my use of -Wimplicit-fallthrough

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>